1. Field of the Invention
The present invention concerns a method, device and computer program product for the purpose of filtering an EMG signal out of a raw signal, i.e., for extracting an EMG signal from a raw signal that contains the EMG signal, among other signal components.
2. Description of the Prior Art
Sensing of EMG-signals in a patient's diaphragm by placing a catheter with a number of electrodes in the esophagus is a known technique, which is described in, among others, U.S. Pat. No. 5,671,752. The EMG-signals received can be used in connection with mechanical ventilation of patients, which among others is described in U.S. Pat. No. 5,820,560 and WO 98/48877.
Sensing of EMG-signals from the diaphragm can even take place outside the body with electrodes placed on the patient, such as described in e.g. U.S. Pat. No. 4,248,240.
If a catheter with electrodes is guided down in the esophagus the electrodes lie on both sides of the diaphragm and at different distances therefrom. Each electrode's position relative to the diaphragm is normally not known and furthermore can vary when the patient breathes or moves in another way.
Since the EMG-signal from the diaphragm is relatively weak, in particular compared to interferences from EKG, a continuous desire is to in the best way attain the highest quality possible signal handling of the raw signal which the sensors detect. This is evident even in WO 01/03579. In WO 01/03579 it is assumed that the electrodes' location in relation to the center of the diaphragm is known. Then the electrodes are weighted based on location and symmetry, in which the EKG signal is taken into account in a conventional way.
Known methods for compensating for the EKG signal include, among others, using a band pass filter which filters out the frequencies where the EKG signal normally appears. It is also known to measure the EKG signal separately and then remove an equivalent signal from the measured EMG signal.
None of these methods takes into account the actual disturbance the EKG signal creates in a particular measuring situation in a particular patient. This disturbance also varies with time.
The known methods also fail to consider that the electrodes location relative to the diaphragm often varies during a single measurement.